Dissertation/Thesis Abstract

Investigating the Impacts of Climate, Hydrology, and Asian Monsoon Intensity on a 13 kyr Speleothem Record from Laos
by Yang, Hongying, Ph.D., University of California, Irvine, 2016, 185; 10027824
Abstract (Summary)

I present a high- resolution record of Southeast Asian Monsoon (SEAM) evolution compiled from δ18O measurements conducted on five U-Th dated speleothems from Tham Mai Cave in northern Laos (20.75N, 102.65E), a key site at the interface between the Indian and East Asian monsoon systems. The speleothem oxygen isotope records are tied to robust uranium-series dates and indicate the records span from 0.79 to 13 kyr BP with sub-decadal resolution. During the Holocene, the Tham Mai speleothem δ18O records are characterized by lower values during the early to mid-Holocene with increasing values towards the late Holocene. This is similar to trends seen throughout the Asian monsoon region, reflecting the strong insolation control on monsoon strength and ITCZ position. The Younger Dryas is characterized by an abrupt δ 18O increase and is synchronous with the even observed in Chinese speleothem records and Greenland ice cores within age uncertainties. This suggests that the SEAM weakened in sync with high-latitude abrupt cooling events.

Four speleothems from Tham Mai cave grew contemporaneously from 4,300 years BP to 9,000 years BP. These four samples show a similar δ 18O pattern, despite a 1.2‰ between sample δ18 O variability is observed. A lumped parameter forward model method (KarstFor model) is used to assess to which extent this 1.2‰ discrepancy can be attributed to hydrological variability. Results suggested that this 1.2‰ discrepancy can be generated due to hydrological variability within one cave.

To better interpret interannual δ18O variability in high-resolution oxygen isotope records in the Asian Monsoon region, I utilize existing simulations from a spectrally nudged isotope-enabled general circulation model (IsoGSM) to investigate the climatic controls on δ18 Op at four cave locations along the Asian monsoon region. Results show that δ18Op at the four cave sites reflects large-scale ocean-atmosphere processes, instead of local precipitation amount. Spatial correlation with vertical wind shear indicates that δ18Op at all sites is significantly related to monsoon strength and Walker circulation. The spatial correlations with SST and precipitation, suggest that the ENSO likely does play a role and that central Pacific type El Niño events influence precipitation δ18O in Oman and northern Laos, in particular.

Indexing (document details)
Advisor: Johnson, Kathleen R.
Commitee: Southon, John, Yu, Jin-Yi
School: University of California, Irvine
Department: Earth System Science
School Location: United States -- California
Source: DAI-B 77/07(E), Dissertation Abstracts International
Subjects: Paleoclimate Science, Atmospheric sciences, Geochemistry
Keywords: Asian monsoon, Holocene, Karst hydrology, Oxygen isotopes, Paleoclimate, Speleothems
Publication Number: 10027824
ISBN: 9781339529011